Pardon for asking, but what non-Greek laymen do with Greek Horologion?
Non-Greek laymen with a good command of Greek, such as Cyrillic, would have access to the original text of the daily offices they pray at home and hear at church. If an equivalent book does not exist in the native languages of said non-Greek laymen, or if those languages could not accurately convey the poetry or meaning of the original (I dont know about Dutch or Finnish, but that is certainly the case with Norwegian), access to the original would be of great benefit. Moreover, if said non-Greek laymen happen to belong to a parish where Greek is the primary liturgical language, they would be able to familiarise themselves with, and perhaps even memorise, the texts they hear in church to allow for a greater level of comprehension and participation.
Funny thing is that had he asked "Where could I get a copy of the HTM or Jordanville Horologia" I very much doubt anyone would ask what a Dutchman was doing with an English prayerbook